1. Field of the Invention
The present invention relates generally to nonreciprocal optical devices, and more particularly to arrays of nonreciprocal devices, such as isolators and circulators, integrated on a common planar optical substrate.
2. Description of the Prior Art
Nonreciprocal optical devices, such as optical isolators and optical circulators, are essential components of optical communication systems. Optical isolators pass light propagating in a forward direction while inhibiting the propagation of light in a backward direction. Optical circulators enable the routing of light from one optical fiber or waveguide to another based upon the direction of light propagation.
Commercially available nonreciprocal optical devices generally take the form of individual (non-integrated) assemblies of bulk optical components. For example, optical isolators typically utilize a GRIN lens attached to an input fiber to collimate the input light. The light is then passed through a series of polarization and Faraday rotation components and subsequently recaptured by a second GRIN lens that recouples the light onto an output fiber. Manufacturing of such isolators involve numerous assembly and manufacturing steps (most of which must be performed manually), resulting in high costs and limitations in production volume. The growth and increasingly price-competitive character of the fiber optic equipment industry has created a demand for low-cost nonreciprocal devices which may be manufactured in large volumes using automated assembly techniques. A particularly strong demand exists for array architectures, in which plural isolators or other nonreciprocal devices are integrated into a single structure.
U.S. Pat. No. 5,706,371 to Pan (“Optical Isolator Array Device”) presents one example of an isolator array architecture. The Pan device consists of corresponding input and output arrays of optical fibers disposed in V-grooves formed on one surface of a supporting substrate. An isolator subassembly, comprising a strip of Faraday material sandwiched between strips of birefringent crystal material, is fixed within a transverse trench formed in the substrate between the input and output optical fiber arrays. Light leaving the input fibers is collimated (either by GRIN lenses located proximal to the fiber endfaces or by thermally expanded cores) and directed onto the isolator subassembly. The receiving ends of the output fibers are provided with collimating elements (GRIN lenses or thermally expanded cores) to couple light transmitted from the corresponding input fibers through the isolator subassembly.
The approach described in the aforementioned Pan patent does offer certain advantages over existing single-channel designs, but has several problems associated with its implementation. These problems include a need to utilize non-standard fibers having thick (>200 μm) claddings to prevent excessive losses resulting from the presence of a sizable evanescent field at the cladding outer surface; processing and induced mechanical fatigue issues associated with thermal expansion of the fiber cores, and; difficulty in automating the placement and alignment of the optical fibers and any separate collimating elements (e.g., GRIN lenses). These and other problems associated with the Pan approach may significantly raise manufacturing costs and compromise device performance. There remains a need in the art for an array-based based nonreciprocal device which is well-suited for high-volume manufacture by automated methods, and which may be produced relatively easily and inexpensively.